Memoirs of Museum Victoria 77: 63–104 (2018) Published 2018
1447-2554 (On-line)
https://museumvictoria.com.au/about/books-and-journals/journals/memoirs-of-museum-victoria/
DOI https://doi.org/10.24199/j.mmv.2018.77.05

Recognising variability in the shells of argonauts (Cephalopoda: Argonautidae): thekey to resolving the taxonomy of the family
Julian K. Finn
Sciences, Museums Victoria, GPO Box 666, Melbourne, Victoria 3001, Australia. Email: jfinn@museum.vic.gov.au

Abstract inn, J.K. 2018. Recognising variability in the shells of argonauts (Cephalopoda: Argonautidae): the key to resolving theFtaxonomy of the family. Memoirs of Museum Victoria 77: 63–104.

Argonauts (Cephalopoda: Argonautidae) are a family of pelagic octopuses that are most commonly recognised by thebeautiful white shells of females (known as paper nautiluses), prized by beachcombers the world over. Taxonomic delineationof the group has historically relied exclusively on features of the shells of females and has resulted in more than 50 speciesnames being coined worldwide. This approach has created considerable confusion in the taxonomy of the family because
argonaut shells are not true molluscan shells and display considerable variation in form. This study closely examined a large
number of argonaut shells from museum collections throughout the world. Two types of shell formation that had beenpreviously attributed to separate argonaut species were recognised within individual shells. It is proposed here that thedifferent shell forms reflect the effects of ecological or biological factors or events, often manifesting as dramatic changes
in shell growth and shape within the development of an individual shell. The resulting combinations of shell formation types
clearly explain both the extreme variation observed across large numbers of argonaut shells and the high number of nominal
species names coined in the past. Researchers coining new fossil argonaut species based solely on shell characters areadvised to proceed with caution. This study supports parallel morphological and molecular research recognising theexistence of only four extant argonaut species worldwide: Argonauta argo, A. hians, A. nodosus and A. nouryi.

Keywordsp aper nautilus, Argonauta argo, Argonauta hians, Argonauta nodosus, Argonauta nouryi, Coleoidea, Octopoda, shellmorphometrics

Introduction
Argonauts (Cephalopoda: Argonautidae) are a family of
pelagic octopuses that inhabit tropical and temperate oceansof the world (fig. 1). Derived from benthic octopus ancestors,
argonauts have departed the sea floor to carry out their entire
life cycle in the open ocean (Young et al., 1998). Argonauts aremost widely recognised by the beautiful white shells of
females that are commonly known as paper nautiluses and are
prized by beachcombers the world over. These shells function
as both brood chambers for the females’ eggs (Naef, 1923) andhydrostatic structures by which female argonauts are able toattain neutral buoyancy (Finn and Norman, 2010).
Images of argonaut shells have a long history, adorning
artefacts dating back to Minoan civilisations (3000–1050 BC;
Walters, 1897; Mackeprang, 1938; Hughes-Brock, 1999) andfeaturing in the earliest conchological works (e.g. Rumphius,
1705; Argenville, 1742; Gualtieri, 1742; Seba, 1758; Martini,1769). By contrast, the identity of the occupant of the shell(i.e. the argonaut) has remained largely unknown ormisinterpreted. For example, in the early 1800s it was widelybelieved that the octopus commonly found in the argonautshell was not the rightful owner, but was a parasite having

devoured the original occupant (Sowerby and Sowerby,
1820–1825; Broderip, 1828).
In the absence of knowledge about the animals that createdthe shells, a taxonomic system that relied completely on shellfeatures arose for the family. Variations in shell shape andappearance formed the basis of new species descriptions,giving rise to 53 species names and 11 subspecies names
worldwide (Sweeney and Young, 2004).
At the core of this taxonomic system is the issue thatargonaut shells display a considerable degree of variability.
This variability has been observed across shells produced byindividuals of the same species (Voss and Williamson, 1971)
and even between opposing faces of the same shell (Cotton andGodfrey, 1940; Trego, 1992). This variability is likely to be
exacerbated by the female argonaut’s ability to repair (Power,
1856; Hoyle, 1886; Boletzky, 1983; Trego, 1993) and completely
rebuild the shell (Holder, 1909a, 1909b; Alliston, 1983).
Argonaut shells are not true molluscan shells. Unlike theshells of other molluscs (e.g. gastropods), argonaut shells are
not produced by the derivatives of the shell field (the mantle
epithelium responsible for shell secretion in other molluscs;
see Kniprath, 1981). In argonauts, the shell field disappears
during embryonic development (Kniprath, 1981). The argonaut

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J.K. Finn

Figure 1. Live female argonaut (Argonauta argo) observed swimming close to the ocean surface and holding her white paper nautilus shell thatfunctions as a brood chamber for the female’s eggs and as a hydrostatic structure for maintaining neutral buoyancy.

shell is a secondary calcium carbonate structure secreted fromwebs on the distal ends of the female argonaut’s first (dorsal)
arm pair.
Female argonauts commence forming shells approximately
12 days after hatching (A. argo: Power in Roberts, 1851; Powerin Catlow, 1854) at a size of approximately 5–7 mm mantle
length (A. argo: Jatta, 1896; Naef, 1923; A. hians: Nesis, 1977;
A. nouryi: Finn, 2009). The initial shell is formed without
sculpturing (Jatta, 1896). By the time the female argonautreaches 10 mm mantle length, the shell (which is now 14 mm
in length) is fully formed (A. hians: Nesis, 1977). The webs on
the female’s dorsal arms overlap the edge of the shell and add
to it as the female grows. Irregularities in the lay of the web
along the shell edge is presumed to cause the undulations inthe surface of the shell, visible as radiating ridges (or ribs) infully formed shells (Mitchell et al., 1994).
Once the female argonuat reaches maturity, she lays long
strands of eggs that are attached to the internal central axis of
the shell. Female argonauts are continuous spawners (Boletzky,
1998; Rocha et al., 2001; Laptikhovsky and Salman, 2003) with
asynchronous ovulation and monocyclic spawning (i.e. egglaying occurring over an extended and continuous spawningperiod in relation to the animal’s life; Rocha et al., 2001).
Spawning is thought to extend over several months (Boletzky,1998) and based on published counts, proposed spawning
frequencies and proposed spawning durations, it has beensurmised that the potential fecundity of a female A. argo could
exceed one million eggs (Laptikhovsky and Salman, 2003).
To stabilise argonaut taxonomy, the aim of this study was
to examine the inter- and intra-specific variation in argonautshell shape. Four key species (identified from morphologicalstudies; see Finn, 2013, 2016) and shells at the centre oftaxonomic confusion for these species were targeted. Thesetarget groups were the Argonauta nouryi/cornutus complex,
the A. hians/boettgeri complex, the A. nodosus/tuberculatus
complex and A. argo. This study supports parallel
morphological and molecular research recognising theexistence of only four argonaut species worldwide: A. argo
Linnaeus, 1758; A. hians [Lightfoot], 1786; A. nodosus[Lightfoot], 1786; and A. nouryi Lorois, 1852.
Materials and methods
More than 1500 argonaut shells were examined over thecourse of this project. Most of the shells examined reside inmuseum collections within Australia, South Africa, Europe
and the United States. Institutions visited include: AustralianMuseum, Sydney, Australia (AMS); Academy of Natural

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Argonaut shell variability
Sciences, Philadelphia, USA (ANSP); The Natural History
Museum, London, UK (BMNH); Muséum National d’Histoire
Naturelle, Paris, France (MNHN); Museums Victoria,
Melbourne, Australia (NMV); Queensland Museum, Brisbane,
Australia (QMB); South African Museum, Cape Town, South
Africa (SAM); South Australian Museum, Adelaide, Australia(SAMA); Santa Barbara Museum of Natural History, Santa
Barbara, USA (SBMNH); Tasmanian Museum and Art
Gallery, Hobart, Australia (TMAG); National Museum of
Natural History, Smithsonian Institution, Washington, USA(USNM); Western Australian Museum, Perth, Australia(WAM). Material loaned from the Museum and Art Gallery of
the Northern Territory, Darwin, Australia (NTM) was
examined at NMV.
While all shells examined ultimately helped in theformation of ideas and an understanding of shell shapevariation, two large collections were pivotal in enablingargonaut shell variability to be interpreted.
The first lot, is a large collection of beach-cast argonautshells collected by Andrés Gonzalez-Peralta (Departamento
de Biologìa Marina, Universidad AutÛnoma de Baja
California Sur, MEXICO) on the beach at El Mogote, La Paz,
Baja California Sur, Mexico, North America, 24° 10' 00" N,
110° 24' 00" W, during the winters of 2000 and 2005. Theseshells are lodged in the collection of SBMNH with thefollowing registration numbers: 172 Argonauta shells collected
on 15 January 2000 – SBMNH 345766 (93 shells), SBMNH
345767 (15 shells) and SBMNH 345768 (64 shells); 92
Argonauta shells collected on 31 January 2005 – SBMNH
357476 (77 shells) and SBMNH 357475 (15 shells).
The second lot was obtained by chance when researchers
on a research expedition off Rowley Shoals, Western Australia
left a pelagic trawl net in the water while steaming between
two stations: north-east of Mermaid Reef (Stn. 10, 17° 23' S,
118° 52' E) and south-west of Imperieuse Reef (Stn. 11,
16° 53' S, 119° 53' E). This occurred on board the FVCourageous on 18–19 August 1983. On recovering the net,
researchers P. Berry and N. Sinclair were surprised to find 73
female argonauts with intact shells. Two specimens werelodged in NMV while the remainder were retained by WAM.
The collection records of these lots are as follows: 71 femaleargonauts – WAM S31520; 2 female argonauts – NMV F87104.
Shell terminology and measurements follow Finn (2013).
The opening of the shell is termed the aperture while the left
and right sides of the shell are termed lateral faces. An
extension of the axial thickening beyond the surface of thelateral face of the shell is termed an ear. The lateral faces areadorned with ribs radiating from the central axis of the shell
towards the keel. Ribs may be smooth (i.e. continuous) or
tuberculated (i.e. consisting of raised separate tubercles). The
keel is bordered by two opposing rows of keel tubercles. The
keel surface may be concave, straight or convex. The presenceof tubercles on the keel surface is known as inter-keel
tuberculation. To allow quantitative comparison of a large
number of shells, a set of standard measurements was taken.
These measurements included: shell length (ShL), maximum
length of shell (note that P indicates that the ShL measurement
was taken from a scaled digital photograph of the shell, notdirectly from the shell); shell weight (ShW), weight (grams) of
dry shell; shell breadth (ShB), maximum breadth of the shell;
rib count (RC), number of ribs adorning a single lateral face,counted around the keel and aperture edge; ear width (EW),
external measurement between lateral tips of opposing ears;
aperture length (ApL), internal distance from the axial
thickening to the ventral keel surface; aperture width (ApW),
internal measurement between two opposing lateral walls at
widest point; keel width (KW), external measurement of keel
at ventral most position; keel tubercle count (KTC), number ofkeel tubercles counted around a single face (see fig. 2).
Scatter plots of measurements against ShL were used toassess differences across large numbers of shells. Regression
lines were plotted using Microsoft Excel for Mac 2011.
Analysis of covariance (ANCOVA) was performed using
Systat 13.2 to assess the significance of the difference between
the slopes of the regression lines.
Scanning electron microscopy was used to examine shell
microstructure and allow accurate measurement of shell
thickness. Shell sections were placed in a sonicator bath forshort periods (5–10 seconds) to dislodge any debris, allowed toair-dry, then placed onto double-sided carbon tabs (Ted Pella,
Redding) and sputter coated with gold. Scanning electron
micrographs were taken using a Zeiss EVO 40 XVP (Zeiss,
Cambridge) housed at SBMNH.
Where female argonauts could be definitively linked withshells, a set of soft body measurements was taken following Finn
(2013). These measurements included: dorsal mantle length(DML), length from posterior tip of mantle to furrow between
mantle edge and base of first arms; mantle width (MW), lateralwidth of mantle at widest point; head width (HW), lateral widthof head measured between the opposing eye surfaces; arm length(AL), length of arm from the edge of the mouth to arm tip,measured along the face of the arm using a piece of string (forarms 2–4); funnel length (FL), distance from the anterior tip ofthe funnel to the posterior medial margin. The relationshipbetween features of female argonauts and their shells were
examined using scatter plots and linear regression.
Results
Argonauta nouryi Lorois, 1852; the A. nouryi/cornutus
complex
In spring each year, small argonauts wash up in large numbers
on beaches in the southern Gulf of California (GonzalesPeralta in Saul and Stadum 2005). These small argonauts are
regularly attributed to two species: A. nouryi Lorois, 1852 andA. cornutus Conrad, 18541 (Garcia-Dominguez and CastroAguirre 1991; Gonzales-Peralta 2006).
A. nouryi was described by Lorois in 1852. The
identification of this species resides solely in features of theshell, which is described as elliptical with numerous finelateral ribs and weak keel tubercles. Fig. 3 incorporates a
1 A third large form also washes up on southern Gulf of California
beaches in spring and is regularly attributed to the species A. pacificus,
a synonym of A. argo; see Finn (2013) for details.

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J.K. Finn

Figure 2. Argonaut shell measurements and terminology, following Finn (2013): a, Argonauta nodosus aperture view (NMV F164695);b, A. nodosus lateral view (NMV F164695); c, A. argo aperture view (WAM S31503); d, A. nouryi aperture view (SBMNH 345766, specimen#074); e, A. nouryi aperture view (SBMNH 345768, specimen #109). Abbreviations: ApL = aperture length; ApW = aperture width; EW = ear
width; KW = keel width; ShB = shell breadth; ShL = shell length. Illustrations: R. Plant.

Argonaut shell variability

67

Figure 3. Comparison of a shell from the examined SBMNH lot with an illustration taken from the original description of Argonauta nouryi
Lorois, 1852: a, reproduction of the illustration from the original description of A. nouryi Lorois, 1852, plate 1, fig. 5; b, illustrations of a shell
matching the description of A. nouryi taken from the examined lot (shell #109, 66.5 mm shell length, SBMNH 345768). Illustration: R. Plant.
Scale bar = 1 cm.

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J.K. Finn

Figure 4. Comparison of a shell from the examined SBMNH lot with the type specimen and illustrations taken from the original description of
Argonauta cornutus Conrad, 1854: a, reproduced illustration taken from the original description of A. cornutus Conrad, 1854, plate 34, fig. 2; b,
photographs of the type specimen illustrated in the original description (58.6 mm shell length, ANSP 63496; please note, the original description
illustrations mirror the characters of the shell, most likely due to the engraving and printing process of the era); c, illustrations of a shell matching thedescription of A. cornutus taken from the examined lot (shell #74, 65.0 mm shell length, SBMNH 345766). Illustration: R. Plant. Scale bar = 1 cm.

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Argonaut shell variability
reproduction of the illustration presented by Lorois, 1852
(plate 1, figure 5), and illustrations of a shell from the Gulf ofCalifornia that is consistent with the original description (shell#109, SBMNH 345768). According to Keen (1971) “the ‘shell’is more elliptical than that of A. cornutus, with only the earlypart of the coil moderately well tinged with brown along thewide and weak tuberculate keel. The surface is delicately
ribbed and has a finely granular texture” (p. 895). Voss (1971)
believed that “Argonauta nouryi is a distinctive species […].
The shells are longer than in any other species of Argonauta,
the ribs are more numerous, there are no distinct tubercles
marking the edges of the carinal area; the carina is wide, very
convex, and covered by numerous, small, blunt tubercles
formed by the crisscrossing of the ribs” (p. 32).
Argonauta cornutus was described by Conrad in 1854. The
identification of this species also resides solely in features of theshell, which is described as having a broad keel, large keel
tubercles and large ears. Fig. 4 incorporates a reproduction of
the illustration presented by Conrad, 1854 (plate 34, figure 2),
photographs of the type specimen (ANSP 63496) andillustrations of a shell from the Gulf of California that is
consistent with the original description (shell #74, SBMNH
345766). According to Keen (1971), “the surface of theyellowish-white ‘shell’ is finely granular, the spines and part ofthe spire dark brown, the keel relatively broad, and the two longaxial expansions suffused with purplish brown” (p. 894). Voss
(1971) summarised that “Argonauta cornutus seems best
characterised by the few radial ribs, the presence of fine sharp
tubercles or papillae over the sides of the shell, the few rather
sharp, large carinal tubercles on each side, the convex carinal
surface, and the few, large, blunt tubercles on the carinal surface
between the two rows of carinal boundary tubercles” (p. 32).
The distributions of these two species are reported tooverlap, with A. cornutus known from the Gulf of California
to Panama and A. nouryi being widespread in the equatorial
Pacific, ranging from the west coast of Southern California to
Peru (Keen 1971).
A mixed lot
As described in the Materials and Methods section above, the157 shells in the collection at SBMNH were collected on thesame beach in Baja California on the same day. These shells
had previously been identified as representing both A. cornutus
and A. nouryi and were registered accordingly: SBMNH
345766, Argonauta cornutus 93 shells; SBMNH 345768,
Argonauta nouryi, 64 shells.
Initial examination of the lots indicated that the shells had
been attributed to either A. cornutus or A. nouryi based on thepresence or absence of ears – a character historically attributed
to only A. cornutus. Further examination of the lot revealed that
separation of the shells into two distinct groups (i.e. either A.
cornutus or A. nouryi) was not as straightforward as first thought.
While some shells within the lot displayed all the characters
associated with either A. cornutus or A. nouryi, the lot also
appeared to contain shells with combinations of the attributes of
the two shell types. To illustrate this variation, three shells ofsimilar size but varied appearance were selected. Fig. 5 presents
photographs of these three shells from multiple perspectives:•

 hell #74 (SBMNH 345766), cornutus-type voucher (fig.
S
5a, i–iv and fig. 4c). Shell morphometrics: ShL 65.0; ShW
4.0; ShB 40.7; RC 45; EW 58.1; ApL 45.9; ApW 28.4; KW
15.6; KTC 27.

•

 hell #42 (SBMNH 345766), intermediate voucher (fig.
S
5b, i–iv). Shell morphometrics: ShL 61.2; ShW 3.1; ShB
36.4; RC 47; EW 36.1; ApL 43.1; ApW 30.9; KW 14.0;
KTC 32.

•

 hell #109 (SBMNH 345768), nouryi-type voucher (fig.
S
5c, i–iv and fig. 3b). Shell morphometrics: ShL 66.5; ShW
2.4; ShB 39.9; RC 54; EW (28.3); ApL 48.8; ApW 32.5;
KW 15.8; KTC 54.

While it would be straightforward to attribute shell #74 (fig.
5a) to A. cornutus Conrad, 1854, and shell #109 (fig. 5c) to A.
nouryi Lorois, 1852, placement of shell #42 (fig. 5b) presents
problems. While shell #42 possesses the aperture shape of A.
cornutus, it lacks its protruding ears. While shell #42 possesses
the keel tuberculation and reduced ventral keel tubercles of A.
nouryi, its dorsal keel tubercles are large and pronounced.
To determine whether there was a significant difference
between eared and earless shells within the lot, a quantitative
approach was undertaken. All intact shells within the lot wereindividually measured and weighed. All shells were designatedas being either eared or earless based on the relative EW and
ApW measurements. Because EW is an external measurement(i.e. measured across the extremities of the opposing ears) and
ApW is an internal measurement (i.e. measured between thelateral walls of the shell), 1.0 mm was added to the ApW toaccommodate for the thickness of the lateral walls of the shell.
Shells were classified as follows:•eared = EW > ApW + 1.0 mm (103 shells)•

earless = EW ≤ Apw + 1.0 mm (35 shells).

Scatter plots were generated to compare eared and earlessshells for all measured characters. Characters of primaryinterest were those previously reported to distinguish A.
cornutus and A. nouryi.
Shell shape. The most universally recognised character of A.
nouryi is reportedly the elliptical shape of the shell: “The
whorls increase in size very rapidly and the last is very
elongate. Viewed laterally it is much shallower than is usual in
the genus” (Robson, 1932, p. 198). The shells are said to be“more elliptical than that of A. cornutus” (Keen, 1971, p. 895)
and “longer than in any other species of Argonauta” (Voss,
1971a, p. 33).
To investigate variation in shell shape across the lot, ShB
was plotted against ShL (fig. 6). Probability plots indicate thatboth ShB and ShL follow normal distributions. An ANCOVA
was used to determine if the slopes of the linear regression
lines, generated for eared and earless shells, were the same ordifferent. Shell type (eared or earless) was the independent
variable, ShB the dependent variable and ShL the covariate.
The ANCOVA revealed that the slopes of the regression lines
are not equal and hence a significant difference in shell shapeexists between eared and earless shells (F(1, 136) = 5.58, p = 0.02).

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J.K. Finn

Figure 5. Three similarly sized shells of varied appearance selected from the examined SBMNH lot: a–c, three similarly sized shells of varied
appearance selected from the single lot collected at El Mogote, La Paz, Baja California Sur, Mexico (24° 10' 00" N, 110° 24' 00" W) on 15 January
2000; a, shell #74 (65.0 mm shell length, SBMNH 345766) assigned the name cornutus-type voucher; b, shell #42 (61.2 mm shell length, SBMNH
345766) assigned the name intermediate voucher; c, shell #109 (66.5 mm shell length, SBMNH 345768) assigned the name nouryi-type voucher;i–iv, multiple perspectives of each shell; i, left lateral view; ii, anterior aperture view; iii, posterior keel view; iv, ventral view. Scale bar = 1 cm.

Argonaut shell variability
Rib count. Argonauta nouryi shells are reported to have moreribs than A. cornutus shells: the ribs in A. nouryi are “morenumerous” than in other species of Argonauta, while A. cornutus
is reported to have “few radial ribs” (Voss, 1971, p. 32–33).
To investigate variation in the number of ribs per shell
across the lot, RC was plotted against ShL (fig. 7). Probability
plots indicate that both RC and ShL follow normal distributions.
An ANCOVA was used to determine if the slopes of the linear

71
regression lines, generated for eared and earless shells, were
the same or different. Shell type (eared or earless) was theindependent variable, RC the dependent variable and ShL thecovariate. The ANCOVA revealed that the slopes of theregression lines are not equal and hence a significant differencein the number of ribs per shell does exist between eared andearless shells (F(1, 136) = 21.2, p < 0.001).
Other features. To investigate the full range of quantifiable
shell characters across the lot, scatter plots were similarly
generated to investigate KTC, ApL, ApW and KW.
Keel tubercle count. To investigate variation in the number ofkeel tubercles per shell across the lot, KTC was plotted against
ShL (fig. 8). Probability plots indicate that both KTC and ShL
follow normal distributions. An ANCOVA was used to determine
if the slopes of the linear regression lines, generated for eared
and earless shells, were the same or different. Shell type (earedor earless) was the independent variable, KTC the dependent
variable and ShL the covariate. The ANCOVA revealed that theslopes of the regression lines are not equal and hence a significant
difference in the number of keel tubercles per shell does exist
between eared and earless shells (F(1, 136) = 51.66, p < 0.001).

Figure 6. Variation in shell shape across the examined SBMNH lot.
Scatter plot of shell breadth (ShB) against shell length (ShL) for the
single shell lot collected at El Mogote, La Paz, Baja California Sur,
Mexico (24° 10' 00" N, 110° 24' 00" W) on 15 January 2000 (SBMNH
345766 & 345768). Eared shells (solid circles) and earless shells (opencircles) distinguished. Linear regression lines for eared shells (dashed)and earless shells (dot dashed) with corresponding equations andcoefficients of determination (i.e. R2 values) presented.

Figure 7. Variation in rib number across the examined SBMNH lot.
Scatter plot of rib count (RC) against shell length (ShL) for the single
shell lot collected at El Mogote, La Paz, Baja California Sur, Mexico(24° 10' 00" N, 110° 24' 00" W) on 15 January 2000 (SBMNH
3045766 & 345768). Eared shells (solid circles) and earless shells(open circles) distinguished. Linear regression lines for eared shells(dashed) and earless shells (dot dashed) with corresponding equations
and coefficients of determination (i.e. R2 values) presented.

Aperture length. To investigate variation in the length of theshell apertures across the lot, ApL was plotted against ShL (fig.
9). Probability plots indicate that both ApL and ShL follow
normal distributions. An ANCOVA was used to determine if
the slopes of the linear regression lines, generated for eared andearless shells, were the same or different. Shell type (eared orearless) was the independent variable, ApL the dependent
variable and ShL the covariate. The ANCOVA revealed thatthe slopes of the regression lines are not equal and hence asignificant difference in the length of the aperture does exist
between eared and earless shells (F(1, 136) = 18.63, p < 0.001).

Figure 8. Variation in keel tubercle number across the examined
SBMNH lot. Scatter plot of keel tubercle count (KTC) against shelllength (ShL) for the single shell lot collected at El Mogote, La Paz, Baja
California Sur, Mexico (24° 10' 00" N, 110° 24' 00" W) on 15 January
2000 (SBMNH 345766 & 345768). Eared shells (solid circles) andearless shells (open circles) distinguished. Linear regression lines for
eared shells (dashed) and earless shells (dot dashed) with corresponding
equations and coefficients of determination (i.e. R2 values) presented.

72
Aperture width. To investigate variation in the width of theshell apertures across the lot, ApW was plotted against ShL(fig. 10). Probability plots indicate that both ApW and ShL
follow normal distributions. An ANCOVA was used todetermine if the slopes of the linear regression lines, generated
for eared and earless shells, were the same or different. Shell
type (eared or earless) was the independent variable, ApW the

J.K. Finn
dependent variable and ShL the covariate. The ANCOVA
revealed that the slopes of the regression lines are not equal andhence a significant difference in the width of the aperture does
exist between eared and earless shells (F(1, 136) = 4.07, p = 0.046).

Figure 9. Variation in aperture length across the examined SBMNH
lot. Scatter plot of aperture length (ApL) against shell length (ShL) for
the single shell lot collected at El Mogote, La Paz, Baja California Sur,
Mexico (24° 10' 00" N, 110° 24' 00" W) on 15 January 2000 (SBMNH
345766 & 345768). Eared shells (solid circles) and earless shells (opencircles) distinguished. Linear regression lines for eared shells (dashed)and earless shells (dot dashed) with corresponding equations andcoefficients of determination (i.e. R2 values) presented.

Keel width. To investigate variation in the width of the shellkeels across the lot, KW was plotted against ShL (fig. 11).
Probability plots indicate that both KW and ShL follow normal
distributions. An ANCOVA was used to determine if the slopesof the linear regression lines, generated for eared and earlessshells, were the same or different. Shell type (eared or earless)
was the independent variable, KW the dependent variable andShL the covariate. The ANCOVA revealed that the slopes of
the regression lines are equal and hence a significant differencein the width of the keel does not exist between eared and earlessshells (F(1, 136) = 0.87, p = 0.353).
Statistical analysis indicates that significance differencesin shell dimensions was associated with the presence orabsence of ears. Eared shells have significantly lower RC (p <0.001), lower KTC (p < 0.001), shorter ApL (p < 0.001),
increased ShB (i.e. shortened; p = 0.02) and increased ApW (p= 0.046). Earless shells have significantly higher RC (p <0.001), higher KTC (p < 0.001), longer ApL (p < 0.001),
reduced ShB (i.e. elongate; p = 0.02) and reduced ApW (p =0.046). KW was found to not be significantly different between
shell types (p = 0.353).
Historically, the features of eared and earless shell types
have been considered to represent separate species such thatfeatures of eared shells are considered characteristic of A.
cornutus, while features of earless shells are consideredcharacteristic of A. nouryi.

Figure 10. Variation in aperture width across the examined SBMNH
lot. Scatter plots of aperture width (ApW) against shell length (ShL)
for the single shell lot collected at El Mogote, La Paz, Baja California
Sur, Mexico (24° 10' 00" N, 110° 24' 00" W) on 15 January 2000
(SBMNH 345766 & 345768). Eared shells (solid circles) and earlessshells (open circles) distinguished. Linear regression lines for eared
shells (dashed) and earless shells (dot dashed) with corresponding
equations and coefficients of determination (i.e. R2 values) presented.

Figure 11. Variation in keel width across the examined SBMNH lot.
Scatter plot of keel width (KW) against shell length (ShL) for the
single shell lot collected at El Mogote, La Paz, Baja California Sur,
Mexico (24° 10' 00" N, 110° 24' 00" W) on 15 January 2000 (SBMNH
345766 & 345768). Eared shells (solid circles) and earless shells (opencircles) distinguished. Linear regression lines for eared shells (dashed)and earless shells (dot dashed) with corresponding equations andcoefficients of determination (i.e. R2 values) presented.

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Argonaut shell variability
Two types of shell formation

The key to understanding shell variation

Close examination of individual shells revealed that featuresconsidered characteristic of each shell type could occur on asingle shell. While individual shells could display features ofboth eared and earless shell types, the characters did notappear in isolation. Sequential growth sections of the shells
appeared to display all the characteristics of one shell type oranother. For example, the initial component of the shell (thesmallest whorl) may display all the characters historicallyassociated with an A. cornutus shell while the latter component(the larger final whorl) may display all the features associatedwith an A. nouryi shell.
The most dramatic examples were shells that appeared tohave been repaired over the course of the argonaut’s life. Fig.
12 presents photographs of one such shell from lot SBMNH
357476 (52.3 mm ShL). The initial component of the shell
clearly displays the features historically attributed to A. nouryi(numerous fine ribs, reduced keel tubercles and no apparentears), while the later component, following the clear repair
line, displays a transition to features historically attributed toA. cornutus (ribs reduced in number and more pronounced,
keel tubercles reduced in number and of larger size, andinitiation of ears).
The presence of both shell types on a single shell clearly
demonstrates that they represent different types of shellformation, not different argonaut species. This observation is
supported by morphological evidence; despite full examinationof nine female argonauts with shells (six historically identified
as A. cornutus and three A. nouryi), no morphologicalcharacters could be found to separate specimens with different
shell types (see Finn, 2013).
The realisation that the two shell morphs represented two
shell formation types, not two argonaut species, required thatthey be defined independent of previous species association:

The realisation that individual shells may be composed of
combinations of two types of shell formation provided the key
to understanding the huge variation in shell shape across the
single large collection of argonaut shells from Baja California.
Combinations of sequential shell formation could berecognised in all shells and hence their varied appearance
could be understood. Shells were recognised within this single
lot that display a single type of shell formation plus those with
one, two or three transformations between the two shellformation types.
The initial whorl of most of the shells displayed Type 1
formation. Shell #37 displays a single change from Type 1 to
Type 2 shell formation (fig. 13). Shell #72 displays a change
from Type 1 to Type 2 shell formation and then a change backto Type 1 (fig. 14). Shell #41 displays a change from Type 1 to
Type 2 shell formation and then a change back to Type 1 andthen to Type 2 (fig. 15). Damage to shells normally results in aconversion to Type 2 shell formation.
In a transition between shell formation types, ears may beformed or subsumed. This is displayed across many shells
within the lot. For examples, shell #139 displays subsumed
ears as a result of a transition from Type 1 to Type 2 shellformation (fig. 16), while shell #136 displays ear formation,
separate from the axis of the shell, as a result of a transition
from Type 2 to Type 1 shell formation (fig. 17).

• ype 1 shell formation (historically attributed to A.
T
cornutus shells) – formation of ears, few pronounced ribs,
few large keel tubercles, appearance of more pronounced
arch in the keel resulting in a tighter final whorl (i.e.
increased ShB, reduced ApL).

• ype 2 shell formation (historically attributed to A. nouryi
Tshells) – absence of ears, numerous less pronounced ribs,
numerous small keel tubercles, appearance of lesspronounced arch in the keel resulting in the appearance ofa shallower final whorl and elliptical shell (i.e. reduced
ShB, increased ApL).

An important character associated with Type 2 shellformation is inter-keel tuberculation (tubercles on the keelsurface; see fig. 2e). The appearance of inter-keel
tuberculation on the keel of a shell flags a shift to Type 2
shell formation, while a loss of inter-keel tuberculation
signifies a shift to Type 1 shell formation.
Based on this realisation, it became clear that this large lot,
and all other material examined of these shell morphs, belonged
to a single species. Because A. nouryi Lorois, 1852, has datepriority over A. cornutus Conrad, 1854; this study treats A.
nouryi as the available name. See Finn (2013) for full synonymy.

Type material. Available type material for additional species
synonymised with A. nouryi Lorois, 1852, was also examined
for shifts in shell formation type. The holotype of A. dispar
Conrad, 1854 (54.9 mm ShL, ANSP 129978) displays a single
change from Type 2 to Type 1 shell formation (fig. 18). The
holotype of A. expansus Dall, 1872 (80.2 mm ShL [P], USNM
61368), displays two changes – from Type 1 to Type 2 and thenback to Type 1 (fig. 19).
Shell thickness. Preliminary observations suggested that theshell walls of Type 1 formation are thicker than the walls ofType 2 formation. To investigate this phenomenon, a scanningelectron microscope was used to examine variation in shell
thickness across recognisable shell breaks that correspondedwith a switch between shell types (a single damaged shell fromlot SBMNH 357476 was sacrificed). Preliminary results
indicate a reduction in shell wall thickness between Type 1 andType 2 formation. Fig. 20 presents two scanning electron
micrographs displaying a reduction in thickness across a breaksignifying transition from Type 1 to Type 2. Shell thickness on
the lateral face drops from approximately 220 to 140 µm (fig.
20a), while thickness at the keel drops from approximately 275
to 210 µm in this shell (fig. 20b).
A lack of material that could be fragmented for examination
with a scanning electron microscope limited the extent towhich this phenomenon could be investigated. The lots housedin the SBMNH collection are too valuable to be considered forthis style of destructive investigation.
A reduction in shell wall thickness may be related to
producing a larger shell area with less shell material. The
resulting thinner walled shell (Type 2) would therefore consist

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J.K. Finn

Figure 12. Repaired shell displaying components consistent with Argonauta nouryi and A. cornutus: a–d, four perspectives of a single shell (52.3mm shell length, SBMNH 357476) displaying an initial component consistent with A. nouryi Lorois, 1854 (“nouryi”) followed by a subsequent
component consistent with A. cornutus Conrad, 1854 (“cornutus”); a, right lateral view; b, oblique right lateral view; c, anterior aperture view;d, oblique ventral keel view. Dashed line represents repair line separating two visually different components. Scale bar = 1 cm.

Argonaut shell variability

75

Figure 13. Argonauta nouryi shell displaying a single change in shell formation type: a–d, four perspectives of shell #37 (65.5 mm shell length,
SBMNH 345766) displaying a single change from Type 1 (T1) to Type 2 (T2) shell formation; a, left lateral view; b, oblique left lateral view; c,close-up oblique left lateral view; d, posterior keel view. Scale bar = 1 cm.

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J.K. Finn

Figure 14. Argonauta nouryi shell displaying two changes in shell formation type: a–d, four perspectives of shell #72 (55.4 mm shell length,
SBMNH 345766) displaying two changes from Type 1 (T1) to Type 2 (T2) shell formation and back to Type 1; a, right lateral view; b, left lateralview; c, oblique left lateral view; d, close-up oblique left lateral view. Scale bar = 1 cm.

Argonaut shell variabilityof less calcium carbonate and weigh less than an equivalently
sized thicker walled shell (Type 1). The relative weights of thethree shells presented in fig. 5 appear to support this theory.
The Type 1 shell (cornutus-type voucher; 4.0 g) is 1.3 times
the weight of the Type 1/Type 2 shell (intermediate voucher;
3.1 g) and 1.7 times the weight of the Type 2 shell (nouryi-type
voucher; 2.4 g), despite the shells having similar ShL. Weight(g) to length (mm) ratios of the three shells were: 1:16 for theType 1 shell (cornutus-type voucher); 1:20 for the Type 1/Type
2 shell (intermediate voucher); 1:28 for the Type 2 shell(nouryi-type voucher). These ratios suggest that per gram ofcalcium carbonate, Type 2 shell production results in a shell
1.8 times the length of a Type 1 shell.
To investigate this relationship across the lot, ShW wasplotted against ShL with eared and earless shells distinguished(fig. 21). The scatter plot indicates a separation between eared
and earless shells based on weight. This difference wasanalysed statistically to determine significance. Probability
plots indicate that both ShW and ShL follow normal
distributions. An ANCOVA was used to determine if the77
slopes of the regression lines, generated for eared and earlessshells, were the same or different. Shell type (eared or earless)
was the independent variable, ShW the dependent variable andShL the covariate. The ANCOVA revealed that the slopes of
the regression lines are not equal and hence a significant
difference in weight exists between eared and earless shells(F(1, 136) = 86.7, p < 0.001).
Argonauta hians [Lightfoot], 1786; the A. hians/boettgeri
complex
Recognition of shell form transformations in A. nouryi
provided a new perspective on shell variation in another highlyvariable group of small argonauts, the A. hians/boettgeri
complex.
The original description of A. hians [Lightfoot], 1786,
refers to a single image in Rumphius (1705): plate 18, figure B(fig. 22a), designated as a lectotype by Moolenbeek (2008) in
the absence of type material. Shells of A. hians can berecognised by smooth lateral ribs and a keel that increases inwidth with shell growth. Inter-keel tuberculation is absent.

Figure 15. Argonauta nouryi shell displaying three changes in shell formation type: a–b, two perspectives of shell #41 (64.7 mm shell length,
SBMNH 345766) displaying three changes in shell formation type from Type 1 (T1) to Type 2 (T2) shell formation, back to Type 1 and then to
Type 2; a, left lateral view; b, oblique left lateral view. Note that the key to recognising the different shell formation types (challenging in this shell)
is to look for reductions in the size of sequential keel tubercles (that would normally increase in size), a change in the relative distance between keel
tubercles, a change in the ratio of lateral ribs to keel tubercles, and the appearance or disappearance of inter-keel tuberculation. Scale bar = 1 cm.

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J.K. Finn

Figure 16. Argonauta nouryi shell displaying subsumed ears: a–d, four perspectives of shell #139 (72.2 mm shell length, SBMNH 345768)
displaying subsumed ear (E) associated with a shift from Type 1 (T1) to Type 2 (T2) shell formation; a, left lateral view; b, close-up of subsumed
ear, left lateral view; c, close-up of subsumed ear, oblique left lateral view; d, anterior aperture view. The shell added to the aperture edge in Type2 shell formation does not expand the ear, instead subsuming it. The resulting aperture edge is not eared. Scale bars = 1 cm.

Argonaut shell variability

79

Figure 17. Argonauta nouryi shell displaying ear formation: a–c, three perspectives of shell #136 (63.1 mm shell length, SBMNH 345768)
displaying ear (E) formation associated with a shift from Type 2 (T2) to Type 1 (T1) shell formation; a, left lateral view; b, close-up of ear, leftlateral view; c, anterior aperture view. The shell added to the aperture edge in Type 1 shell formation produces a new ear separate from the axisof the shell. The new ear becomes the widest point on the aperture edge. Scale bars = 1 cm.

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J.K. Finn

Figure 18. Holotype of Argonauta dispar Conrad, 1854 (synonym of A. nouryi Lorois, 1852) from the Academy of Natural Sciences, Philadelphia:a–d, four perspectives of A. dispar Conrad, 1854 Holotype (54.9 mm shell length, ANSP 129978) displaying a single change from Type 2 (T2)
to Type 1 (T1) shell formation; a, left lateral view; b, right lateral view; c, anterior aperture view; d, posterior keel view. Scale bar = 1 cm.

Argonaut shell variability

81

Figure 19. Holotype of Argonauta expansus Dall, 1872 (synonym of A. nouryi Lorois, 1852) from the National Museum of Natural History(Smithsonian Institution) Washington: a–c, three perspectives of A. expansus Dall, 1872 Holotype (80.2 mm shell length [P], USNM 61368)
displaying two changes from Type 1 (T1) to Type 2 (T2) shell formation and back to Type 1; a, left lateral view; b, anterior aperture view; c,posterior keel view. Scale bar = 1 cm.

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J.K. Finn

Figure 20. Scanning electron microscope images of Argonauta nouryi shell displaying variation in shell thickness: a–b, scanning electron
microscope images of shell cross-sections (SBMNH 357476) across shell repairs (R) representing a shift from Type 1 (T1) to Type 2 (T2) shellformation; a, lateral face of shell, inner surface facing up; b, keel, outer surface facing up. Scale bars = 1 mm.

83

Argonaut shell variability

prominent in the first half of the shell and may remain large on
the last half or may become considerably smaller” (p. 105).
Two shells are presented as examples:

Figure 21. Variation in shell weight across the examined SBMNH lot.
Scatter plot of shell weight (ShW) against shell length (ShL) for the
single shell lot collected at El Mogote, La Paz, Baja California Sur,
Mexico (24° 10' 00" N, 110° 24' 00" W) on 15 January 2000 (SBMNH
345766 & 345768). Eared shells (solid circles) and earless shells (opencircles) distinguished. Linear regression lines for eared shells (dashed)and earless shells (dot dashed) with corresponding equations andcoefficients of determination (i.e. R2 values) presented.

Argonauta hians has long been recognised as displaying
considerable variation in shell form. Voss and Williamson(1971) noted that “In the series from Hong Kong the sides of
the aperture at the umbilicus range from strongly eared orauriculate with very large few knobs on the keel to specimens
with no trace of auriculation and with rather more numerous,
smaller knobs” (p. 105). They found that “if the 30 shells arelaid out graded from large few knobs and strong auricles tosmaller, more numerous knobs and flat sides there is an evengradation with no breaks or sudden changes” (p. 105). They
concluded that all shells “belong to the same species” (p. 105).
As part of this study, 274 A. hians shells were directly
examined in museum and private collections in Australia,
United States, Europe, South Africa and Japan. Withknowledge gained from examining shells of A. nouryi, allshells from all sites were examined for an abrupt change inkeel tubercle height or ears that had been formed or subsumed
in single shells. Because inter-keel tuberculation is not
expressed in argonaut shells other than A. nouryi, thischaracter could not be used.
Two shell formation types. Shells of A. hians were found to
display two clear shell formation types:•

 ype 1 shell formation – few pronounced ribs, large
Tprominent keel tubercles, formation of ears.

•

 ype 2 shell formation – numerous less-pronounced ribs,
Tsmall and greatly reduced, keel tubercles, absence of ears.

These shell formation types are similar to those expressed in
A. nouryi except that variation in the arch of the shell was notobserved and inter-keel tuberculation was not present.
This variation had been noted by Voss and Williamson(1971) who stated: “The knobs on the keel are very large and• shell from the Philippines (79.6 mm ShL [P], BMNH
A
unreg., “Cuming, i.”) (fig. 23). This shell displays a clear
shift from Type 1 to Type 2 shell formation indicated by areduction in the size and spacing of the keel tubercles, a
reduction in the ratio of ribs to keel tubercles fromapproximately 1.5:1 to 1:1 and ears subsumed.

•

 shell from the North West Shelf, Western Australia (53.0Amm ShL, WAM S31510) (fig. 24). This shell displays ashift from Type 1 to Type 2 shell formation. This transition
occurred when the shell was at a smaller size and hence
the ears are less developed. The resultant aperture shape(fig. 24c) is extremely similar to that observed in Type 2 A.
nouryi shells; see fig. 5c, ii for comparison.

Variation also occurs between the opposing faces of individual
shells, further highlighting the plasticity of shell characters inthis species. A single shell is presented here as an example:•

 shell from Madagascar (60.8 mm ShL, NMV F164734,
A“Madagascar”) displays a large ear on the right side only;
the left side is earless (fig. 25).

The A. hians/boettgeri complex. Small, earless A. hians shells
have regularly been attributed to the species A. boettgeri
Maltzan, 1881 (fig. 22b, c). Smith (1887) outlined the diagnostic
characters of A. boettgeri: “The distinguishing features of thisspecies are the numerous ribs and tubercles, the total absence ofauricular expansions at the sides, its constantly small size, andthe fine granulation (a feature not remarked upon by Maltzan),
which more or less covers the whole surface, producing a dull
non-glossy appearance” (p. 409). Berry (1914) similarly notedthat the shell of A. boettgeri “seems unique in its small size,
compact coil, and the circumstance that the auricular expansion
at the sides of the aperture, so frequently developed in otherspecies of the genus, are here notable only for their entireabsence” (p. 280). Robson (1932) added “the almost invariable
absence of colouring on the carinal knobs” to the distinguishing
characters of A. boettgeri (p. 197). While Smith (1887) concludes
that “the shell of this species must not be confounded withyoung stages of A. hians; the more numerous ribs and tubercles
and the rougher granular surface will separate it” (p. 410).
Unfortunately, this dichotomy is not so straightforward.
Of the 274 A. hians shells examined, 41 can be attributed
to A. boettgeri based on the above description. While it ispossible to select a subset of shells possessing thesecharacteristics, which in isolation appear distinct, examinationof the entire range of material quickly dissolves the parameterson which this subset is based. All features mentioned above
are variable in A. hians: ribs and keel tubercles can be
numerous or scarce, pronounced or reduced, consistent acrossthe shell or variable; ears can be present or absent, produced or
subsumed, expressed on one side of the shell or both; the shellsurface can be granular or smooth, pigmented or white. Twoshells, displaying variation across the growth of the shell, arepresented as examples:

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J.K. Finn

Figure 22. Reproduced illustrations referenced in the descriptions of Argonauta hians [Lightfoot], 1786 and A. boettgeri Maltzan, 1882: a,illustration of A. hians [Lightfoot], 1786, designated as a lectotype by Moolenbeek (2008), Rumphius, 1705: pl. 18, fig. B; b–c, illustrations of A.
boettgeri Maltzan, 1881, featured in the original publication, Maltzan, 1881: 163, pl. 6 fig. 7; b, right lateral view; c. anterior aperture view.

Argonaut shell variability

85

Figure 23. Argonauta hians shell from the Philippines: a–d, four perspectives of an A. hians shell from the Philippines (79.6 mm shell length [P],
BMNH unreg., “Cuming, i.”) displaying a clear shift from Type 1 shell formation (T1) to Type 2 shell formation (T2) indicated by a reduction in
the size and spacing of the keel tubercles, a reduction in the ratio of ribs to keel tubercles (from approximately 1.5:1 to 1:1) and subsumed ears;a, right lateral view; b, anterior aperture view; c, posterior keel view; d, ventro-posterior keel view. Scale bar = 1 cm.

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J.K. Finn

Figure 24. Argonauta hians shell from the North West Shelf, Western Australia: a–d, four perspectives of an A. hians shell from the North West
Shelf, Western Australia (53.0 mm shell length, WAM S31510) displaying a clear shift from Type 1 shell formation (T1) to Type 2 formation (T2)
indicated by a reduction in the size and spacing of the keel tubercles and a reduction in the ratio of ribs to keel tubercles (from approximately
1.5:1 to 1:1); a, right lateral view; b, oblique right lateral view; c, anterior aperture view; d, posterior keel view. Scale bar = 1 cm.

Argonaut shell variability

87

Figure 25. Single eared Argonauta hians shell from Madagascar: a–c, three perspectives of a single eared A. hians shell from Madagascar (60.8mm shell length, NMV F164734); a, left lateral view; b, right lateral view; c, anterior aperture view. Scale bar = 1 cm.  

88•

J.K. Finn shell from the British Museum (76.1 mm ShL [P],A
BMNH unreg., locality unknown, “B698, t.”; fig. 26).
This shell displays an aperture shape and axial region
consistent with the original description of A. boettgeri(fig. 22b, c) yet defies the description of A. boettgeri byshowing signs of possessing ears at an earlier growthstage. While the ears have been subsumed with a shift
from Type 1 to Type 2 shell formation, only the keel
tubercles on the right side show a reduction in size (fig.
26b); the left keel tubercles have remained large (fig. 26a).

• shell from Museums Victoria (25.0 mm ShL [P],A
NMV F164767, locality unknown; fig. 27). This shellwould historically have been attributed to A. boettgeri
due to its small size and distinctive earless aperture.
This shell displays a dramatic change in keel tubercle
size and spacing associated with a shift from Type 2 toType 1 shell formation, thus highlighting the plasticityof these characters.
In the absence of any consistent and definable diagnostic
shell characters (in combination with a lack of diagnostic
morphological characters or distinct distributions; see Finn,
2013), no evidence exists to justify maintaining A. boettgeri asa separate species. Consequently A. boettgeri Maltzan, 1881,is treated here as a synonym of A. hians [Lightfoot], 1786.
Insight from whole animals. As described in the Materials andMethods section above, a single specimen lot of 73 female A.
hians, most with intact shells, exist in the collections of theWestern Australian Museum and Museums Victoria. On initial
examination, it was found that the lot included submature,
mature and spawned (i.e. females with eggs attached to thecentral axis of the shell) individuals. The shells of the spawned
females tended to show a shift to Type 2 shell production in thelast components of the shells (all other shells were composed
entirely of Type 1 shell production). This led to the considerationthat shell shape and transformation may be triggered bychanges in reproductive stage or condition.
To understand the underlying cause of a change in shellformation type at the point of egg laying, a subset of 33 intact
and measurable individuals were selected and fully measured.
The subset included submature, mature and spawned
individuals, with a size range of 13–27 mm DML and 21–36mm ShL. Two larger females, also collected over the NorthWest Shelf, were incorporated into the analysis to expand thesize range (QM Mo77789: 39.9 mm DML and 51.8 mm ShL;
28.7 mm DML and 38.9 mm ShL).
Changes in shell morphometrics relative to animal size. Shelldimensions were plotted against DML to determine if the sizeof the shell relative to the size of the female changes between
submature, mature and spawned individuals. Scatterplots
against DML were generated for ShL, ShB, ApL, ApW, KW
and EW. The scatter plots indicate a linear relationship between
shell dimensions and animal size, with linear regressions
returning coefficient of determination values (i.e. R2 values)
between 0.72 and 0.90 (see Table 1). No discontinuities wereobserved between the three maturity stages.

Table 1. Linear regression equations for scatter plots of shell dimensions (y) against dorsal mantle length (x) for 35 female Argonauta hians from
Australian waters (WAM S31520/NMV 87104/QM Mo77789) including submature, mature and spawned individuals. Corresponding coefficientsof determination (i.e. R2 values) presented.

yx

equation

R²

Shell length (ShL)

Dorsal mantle length (DML)y = 1.0936x + 7.9242

0.8980

Shell breadth (ShB)

Dorsal mantle length (DML)y = 0.8705x + 0.6315

0.8705

Aperture length (ApL)

Dorsal mantle length (DML)y = 0.8889x + 3.4049

0.8882

Aperture width (ApW)

Dorsal mantle length (DML)y = 0.3875x + 9.3432

0.7697

Keel width (KW)

Dorsal mantle length (DML)y = 0.1839x + 4.1154

0.7244

Ear width (EW)

Dorsal mantle length (DML)y = 0.3360x + 11.1327

0.7268

Table 2. Linear regression equations for scatter plots of female argonaut dimensions (y) against dorsal mantle length (x) for 35 female Argonauta
hians from Australian waters (WAM S31520/NMV 87104/QM Mo77789) including submature, mature and spawned individuals. Corresponding
coefficients of determination (i.e. R2 values) presented.

yx

equation

R²

Mantle width (MW)

Dorsal mantle length (DML)y = 0.4166x + 6.6124

0.8156

Head width (HW)

Dorsal mantle length (DML)y = 0.4594x + 4.5374

0.8609Funnel length (FL)

Dorsal mantle length (DML)y = 0.4905x + 3.2751

0.7559Arm length 2 (AL2)

Dorsal mantle length (DML)y = 1.8016x - 1.5714

0.8773Arm length 3 (AL3)

Dorsal mantle length (DML)y = 1.1550x + 5.5401

0.8169Arm length 4 (AL4)

Dorsal mantle length (DML)y = 0.8482x + 5.8121

0.8292

Argonaut shell variability

89

Figure 26. Argonauta hians shell from the British Museum: a–d, four perspectives of an A. hians shell from the British Museum (76.1 mm shelllength [P], BMNH unreg., locality unknown, “B698, t.”) which, while displaying an aperture shape and axial region consistent with the originaldescription of A. boettgeri (fig. 22B, C), shows signs of possessing ears (E) at an earlier stage of growth; a, left lateral view; b, right lateral view;c, anterior aperture view; d, posterior keel view. A shift from Type 1 shell formation (T1) to Type 2 shell formation (T2) is expressed by ears
subsumed and a reduction in keel tubercle size on the right side only. Scale bar = 1 cm.

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J.K. Finn

Figure 27. Shell consistent with description of Argonauta boettgeri from Museums Victoria: a–d, four perspectives of a shell consistent with A.
boettgeri (treated here as a synonym of A. hians [Lightfoot], 1786) from Museums Victoria (25.0 mm shell length, NMV F164767) displaying an
increase in keel tubercle size consistent with a shift from Type 2 shell formation (T2) to Type 1 shell formation (T1); a, left lateral view; b, rightlateral view; c, anterior aperture view; d, posterior keel view. Scale bar = 1 cm.

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Argonaut shell variability

Table 3. Linear regression equations for scatter plots of shell dimensions (y) against shell length (x) for 35 female Argonauta hians from Australianwaters (WAM S31520/NMV 87104/QM Mo77789) including submature, mature and spawned individuals. Corresponding coefficients ofdetermination (i.e. R2 values) presented.

yx

equation

R²

Shell breadth (ShB)

Shell length (ShL)y = 0.7676x - 4.8377

0.9015

Aperture length (ApL)

Shell length (ShL)y = 0.8013x - 2.6968

0.9613

Aperture width (ApW)

Shell length (ShL)y = 0.3369x + 7.0488

0.7752

Keel width (KW)

Shell length (ShL)y = 0.1701x + 2.7255

0.8254

Ear width (EW)

Shell length (ShL)y = 0.2936x + 9.1011

0.7391

Ontogenetic changes in animal morphology. Dimensions andcharacters of the female argonauts were plotted against DML todetermine if the relative proportions of the female changes
between submature, mature and spawned individuals.
Scatterplots against DML were generated for MW, HW, FL and
AL. The scatter plots indicate a linear relationship between
animal dimensions, with linear regressions returning coefficientof determination values (i.e. R2 values) between 0.76 and 0.88
(see Table 2). No discontinuities were observed between thethree maturity stages.
Ontogenetic changes in shell morphometrics. Shell dimensions
and characters were plotted against ShL to determine if relative
shell proportions change between submature, mature andspawned females. Scatterplots against ShL were generated for
ShB, ApL, ApW, KW and EW. The scatter plots indicate a linear
relationship between shell dimensions and characters, withlinear regressions returning coefficient of determination values(i.e. R2 values) between 0.74 and 0.96 (see Table 3). No
discontinuities were observed between the three maturity stages.
The scatter plots provided no evidence of a change in
relative shell and animal proportions between submature,
mature and spawned individuals. If the examined charactersunderwent dramatic transformation with changes in state of
maturity, it was expected that discontinuities would beobserved in the plotted data. It is apparent that the visualchange in shell form observed across this lot was not reflectedin the relative measurements of the individuals measured.
Argonauta nodosus [Lightfoot], 1786; the A. nodosus/
tuberculatus complex
The original description of A. nodosus [Lightfoot], 1786, refers
to a single image in Rumphius (1705): plate 18, figure 1 (fig.
28a), designated as a lectotype by Moolenbeek (2008) in theabsence of type material. Shells of A. nodosus can be recognisedby the presence of lateral ribs composed of separate tubercles.
Two types of A. nodosus shells exist in collections: a finer shellwith more ribs and small rib tuberculations (fig. 29a), and a coarser
shell with fewer ribs and larger rib tuberculations (fig. 29b).
This variation has previously been used as justification for
splitting A. nodosus into two species. Kirk (1885), in recognising
the two forms, generated a new species name for the fine
tuberculated and earless form (A. gracilis) to separate it from thecoarse tuberculated and eared form (known to Kirk, 1885, as A.

tuberculata Shaw). Robson (1932) recognised the two shell types
as varieties, not separate species, stating: “Though the shell of
this species is clearly distinguished from its fellows by the roughtuberculations, there are evidently two well marked varieties –one with very large carinal knobs and coarse sculpture, the otherwith low knobs and fine sculpture” (p. 200). Dell (1952) called
this the “nodosa-tuberculata complex”2 and described it asfollows: “Group 1. The shell is eared laterally and thetuberculations on the ribs are comparatively large – this is what
has been called nodosa. Group 2. The edge of the lip comes off
the previous whorl in an even curve without trace of an ‘ear’. The
tuberculations are much finer and more numerous than in Group
1 – tuberculata” (p. 54). Dell (1952) considered both forms tobelong to a single species.
While both shell varieties are common, individual shells
displaying an obvious shift between fine and coarse shellformation are extremely rare. A single shell from Moreton
Bay, Queensland (109.1 mm ShL, QM Mo14232) displays atransition from fine shell formation to coarse shell formation
at a point of previous damage (fig. 30). While the latercomponent of the shell possesses ears, it is not clear whether
the earlier component was eared or earless. No obvious
changes were noted in shell thickness, curvature of the keel orrelative heights of sequential keel tubercles.
Examination of a large number of A. nodosus shells foundno examples displaying a marked change in keel tubercle
height or ears that had been formed or subsumed. While eared
and earless forms exist, transition between the two types
appeared more gradual than the sudden transformation
documented in smaller species. A shell in the British Museum(109.0 mm ShL [P], BMNH unreg., locality unknown, “B395,e.”) displays an ear on only one side, clearly demonstrating theplasticity of this character in this species (fig. 31).
2 Following Finn (2013) it is necessary to correct the original spellingof A. nodosa to A. nodosus. In accordance with the International Code ofZoological Nomenclature, Article 34.2 “the ending of a Latin or latinizedadjectival or participial species-group name must agree in gender with thegeneric name with which it is at any time combined [Art. 31.2]; if thegender ending is incorrect it must be changed accordingly (the author anddate of the name remain unchanged)” (I.C.Z.N., 1999). As Argonauta ismasculine “from the final noun nauta (a sailor)” (I.C.Z.N., 1999, p. 34)
the species-group name must be changed from the feminine nodosa (-afeminine) to the masculine nodosus (-us masculine).

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Figure 28. Reproduced illustrations referenced in the descriptions of Argonauta nodosus [Lightfoot], 1786 and A. argo Linnaeus, 1758: a,illustration of A. nodosus [Lightfoot], 1786, designated as a lectotype by Moolenbeek (2008), Rumphius 1705, pl. 18, fig. 1; b, illustration of A.
argo Linnaeus, 1758, considered a paralectotype following the designation of a lectotype by Moolenbeek (2008), Rumphius 1705, pl. 18, fig. A.

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93

Figure 29. Coarse and fine Argonauta nodosus shells: a, fine A. nodosus shell from Mayor Is., Bay of Plenty, New Zealand (127.2 mm shelllength, NMV F164784); b, Coarse A. nodosus shell from the Indo Pacific (127.3 mm shell length, NMV F164774). Scale bar = 1 cm.

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Figure 30. Repaired Argonauta nodosus shell from Moreton Bay, Queensland. Left lateral view of repaired A. nodosus shell from Moreton Bay,
Queensland (109.1 mm shell length, QM Mo14232) showing a transition from fine shell formation (Fine) to coarse shell formation (Coarse) at
point of previous damage. Scale bar = 1 cm.

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Argonaut shell variability
Argonauta argo Linnaeus, 1758
An image referenced in the original description of A. argo
Linnaeus, 1758, is considered a paralectotype, with designationof a lectotype by Moolenbeek (2008); Rumphius, (1705) plate
18 figure A (fig. 28b). Shells of A. argo can be recognised by
an extremely narrow keel of consistent width. The keel
tubercles are paired and the lateral ribs are continuous (i.e.
they are not broken into separate tubercles).
Shells of A. argo are extremely consistent in dimensions andsculpturing. The area that has caused the most confusion for
naturalists defining the species has been the aperture edge. A.
argo can display huge variation in the shape of the aperture edge
near the axis. Note the variation in the aperture edge of the twoshells presented in fig. 32. Unlike ear formation, this variation
occurs on the edge of the lateral wall parallel with the longitudinal
axis of the shell; it is not a lateral extension. The expression of thelateral ribs can vary slightly from fine to coarse, suggesting thepresence of two varieties (fig. 32). Transition between fine andcoarse shell formation on a single shell is extremely rare. An
illustrated shell from Monterey, California (81.9 mm ShL,USNM 61374) displays a shift from finer to coarser shellformation at the point of earlier damage (fig. 33). Small A. argoshells can also display laterally protruding ears. A shell from
Venezuela (51.4 mm ShL, USNM 122208) highlights theplasticity of this character, displaying an ear on only the rightside (fig. 34). The varied size and shape of the keel tubercles on
the opposing sides of this shell demonstrate the range ofvariability of these structures in this species.
Discussion
Among molluscs, the shells of small argonaut species (inparticular, A. nouryi) display an unprecedented level of variability.
The extreme forms are so different that it initially seems
incomprehensible that they could be produced by the sameargonaut species. Given this apparent disparity, it is necessary toemphasise that argonaut shells are fundamentally different innature from the true molluscan shells of non-cephalopod
molluscs; they are produced by different structures, for different
reasons and have a different construction.
Shell material laid down by females of small argonautspecies (A. nouryi and A. hians) can take one of two distinct
morphologies. Firstly, the shell can be heavy and thick walledwith prominent sculpture (Type 1 shell formation). The large
corrugations of the lateral walls are displayed as distinct robust
lateral ribs. The thickened keel is defined by two rows of large
and distinct keel tubercles. The axis of the shell projects laterally
to form large ears, providing support to the lateral walls. In thesecond form, the shell can be lightweight and thin walled, withgreatly reduced sculpture (Type 2 shell formation). The
corrugations of the lateral walls are downgraded to fine lateralribs. The convex keel is undefined, with the keel tubercles
diminished to slight projections of the lateral rib extremities.
The axis of the shell is rolled ventrally to join the aperture edge
without lateral projection (i.e. earless).
The shell morphology expressed by a growing femaleargonaut does not follow a predetermined order. Shells of
female A. nouryi demonstrate that females can switch between

the two shell formation types at least three times during
production of a single shell. The initial shell formation type isvariable (it can be Type 1 or Type 2), as is the portion of shelllaid down before switching to another shell formation type.
It is largely impossible to determine the conditions anargonaut was exposed to at the time that it switched shellformation types. The exception is the response to shell breakage.
Individual shells retain evidence of earlier trauma in the form of
repairs and irregularities in shell form. A. nouryi shells almost
invariably display a shift to Type 2 shell formation following
major damage. At the time of shell breakage, an argonaut would
be exposed and vulnerable. As has been observed for A. argo,shell integrity is critical in allowing the argonaut to attainneutral buoyancy, free itself from the sea surface and undertake
rapid horizontal locomotion (Finn and Norman, 2010). In theabsence of shell-aided buoyancy, the female must remain in thewater column by siphon-jetting alone. As such, it would be
imperative for a female argonaut to rebuild her shell as quickly
as possible following any damage. It is assumed that the shift tothinner walled Type 2 shell formation allows the female torebuild her shell more rapidly, spreading the available building
material (calcium carbonate) over a greater area.
The different morphologies expressed in an individual A.
nouryi shell are therefore considered to represent periods ofvaried rate of shell formation. Components of a shell laid
down over longer periods are believed to exhibit thicker walls
and more prominent sculpture (Type 1), while rapidly produced
sections display thinner walls and reduced sculpture (Type 2).
The rate at which the female lays down the shell is believed todetermine the gross morphology of the shell. Based on thispresumption, two hypotheses are raised to explain the variable
shell production rate (expressed as the variable shell formationtype) evident in undamaged, unrepaired shells:•

 ypothesis 1: Rate of shell production correlates directlyHwith animal growth. Three factors are believed toinfluence octopus growth rate: temperature, nutrition, andmaturation or reproduction (see Semmens et al., 2004 fora review). While very little is known about the lives ofargonauts, they are known to occur in the open ocean
spanning huge geographical distributions. This wideranging pelagic existence has the potential to expose them
to a mosaic of food availability and water temperatures.
Encountering a large school of prey or pocket of warmer
water may result in a period of increased growth.
Additionally, reproductive investment (i.e. egg production)
may slow body growth. This hypothesis suggests thatthese periods of varied morphological growth of theanimal are reflected in the gross morphology of the shell.

•

 ypothesis 2: Rate of shell production influenced byH
external factors. The shells of female argonauts, inaddition to providing protection and buoyancy, primarily
function as a case for external brooding of the female’s
eggs. Strings of eggs are suspended from the inner core of
the shell. This strategy requires that the internal volumeof the shell accommodate both the female argonaut andher eggs. This hypothesis proposes that the spaceconstraints associated with commencement of egg

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J.K. Finn

Figure 31. Single eared Argonauta nodosus shell from the British Museum: a–c, three perspectives of a single eared A. nodosus shell from the
British Museum (109.0 mm shell length [P], BMNH unreg., locality unknown, “B395, e.”); a, left lateral view; b, right lateral view; c, anterior
aperture view. Scale bar = 1 cm.

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97

Figure 32. Coarse and fine Argonauta argo shells: a–b, shells of A. argo displaying different degree of sculpturing and variation in the apertureedge; a, fine A. argo shell from off San Clement Island, California (113.3 mm shell length [P], USNM 316580); b, coarse A. argo shell from Baja
California (128.1 mm shell length [P], ANSP 404279). Scale bar = 1 cm.

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J.K. Finn

Figure 33. Repaired Argonauta argo shell from Monterey, California. Repaired A. argo shell from Monterey, California (81.9 mm shell length,
USNM 61374): a, left lateral view; b, oblique left lateral view; c, oblique anterior aperture view. Note change in direction of lateral ribs alongrepair line. Scale bar = 1 cm.

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99

Figure 34. Single eared Argonauta argo shell from Venezuela, South America: a–c, three perspectives of a single eared A. argo shell from
Venezuela (51.4 mm shell length, USNM 122208); a, left lateral view; b, right lateral view; c, anterior aperture view. Scale bar = 1 cm.

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J.K. Finn
spawning triggers an increase in shell production rate.
The gross morphology of the shell therefore displays
intermittent periods of volume constraints as a result ofintermittent spawning or brooding events.

To gain insights into the relationships between features of theargonaut shell and its female occupant, focus was directed at
A. hians. The large single lot of female A. hians (withaccompanying shells) from Western Australia includes threeclasses: submature, mature (unspawned) and spawned
individuals. Examination of the lot revealed that all spawned
individuals (i.e. all individuals with eggs deposited within
their shells) displayed a shift from Type 1 to Type 2 shellformation. No converse arrangements were observed. Basedon this qualitative observation, this large sample appeared tosupport Hypothesis 2; the presence of spawned eggs within theshell causing a space constraint and thus triggering a shift tomore expansive thin-walled shell production. Hypothesis 1
would predict slower body growth of the argonaut associatedwith increased reproductive investment in egg production.
This would predict slower shell production and hence a shift toType 1 formation. This was not observed. Quantitative analysisof argonauts in this lot did not provide further insight. A fullrange of characters of both the animals and shells were
measured. Features of the shell, the female and the shell
relative to the female were compared, plotted and analysedwith linear regression. In all instances, scatter plots indicatedlinear relationships between animal and shell dimensions,with linear regressions returning high coefficients ofdetermination values (i.e. R2 values) with no discontinuities
observed between submature, mature and spawned individuals.
Examination of shells of larger species (A. nodosus and A.
argo) revealed that they are not subject to the extreme variability
in shell form identified in the smaller argonaut species. Whileboth large species display two distinct morphologies (coarse
and fine forms), the two shell types are never expressed asalternations on individual shells. Rare examples of extremely
damaged shells can display a shift from fine to coarse
morphology, but a reversion (i.e. from coarse to fine) was neverobserved. Because the two shell types do not vary in shell wall
thickness or amount of material used, the variation between thetwo shell types appears fundamentally different from thatobserved in smaller species. One possible explanation is thatthe coarser shells of larger species represent reformed shells
produced by large individuals (i.e. new shells constructed toreplace lost or damaged shells), while the finer shells represent
the original shells that are produced as the animals grow.
In the absence of captive rearing studies and sequential
collections of the same argonaut species from the same
location, it is not possible to conclusively support either
hypothesis. Based on limited observational evidence, it is theauthor’s opinion that the variation observed in the shells ofsmall argonaut species is the result of space constraints (i.e.
Hypothesis 2) and independent of argonaut growth. The prime
circumstantial evidence comes firstly from gross differencesin shell occupation between large and small species, andsecondly from the dramatic transformation or reversion
boundaries on the shells of small species.

Gross differences in shell occupation. At commencement of
egg laying, the shells of females of small argonaut species
possess an extremely small amount of available space for egg
storage. Fig. 35a shows a preserved female A. nouryi that had
already commenced egg laying with a DML of 15.2 mm(SBMNH 64369). As can be seen from the image, the space atthe top of the shell where the eggs are to be stored is extremely
small. The shell has barely formed through 90 degrees. Storageof egg-strings within this shell will have a significant impact
on the space available for this small female within the shell.
Fig. 35b shows a female A. hians with a DML of 28.7 mm (QM
Mo77789). Yellow eggs are clearly visible and occupy almost
half of the shell volume. While the shell has developed throughalmost a complete rotation, the volume occupied by the eggs
significantly displaces the female. With the posterior tip of the
mantle firmly against the egg mass, the female is still only
partially within her shell. Note the distance of the eye from theedge of the shell aperture. In the absence of eggs, femaleargonauts typically retract well into their shells with their eyes
at the boundary of the lateral walls. Fig. 36a presents a
photograph of a live female A. hians photographed in an
aquarium (after Sukhsangchan and Nabhitabhata, 2007). Witha large volume of eggs in the initial whorl of the shell, thefemale can only partially retract within. The aperture edge ofthe shell sits posteriorly to the mantle edge and a considerable
distance from the female’s eye.
Displacement of the female from the shell would provide astrong stimulus for rapid shell deposition, resulting in theextended flange-like form of Type 2 shells. Subsequentinterruptions to egg production (or hatching) could explain areturn to full occupancy of the shell and Type 1 shell formation,
as demonstrated in A. nouryi.
The apparent space constraint observed in smaller species
is not evident in larger species. Females of A. nodosus, observed
live, appear uninfluenced by large volumes of eggs held within
their shells. Fig. 36b presents a photograph of a live female A.
nodosus. This female is positioned well within her shell; notethe proximity of the female’s eye to the edge of the shell
aperture. Although not apparent from this photograph, thefemale is carrying a huge volume of eggs. Fig. 36c presents theegg strings revealed on removal of the female from her shell.
It is possible that the increased size of the shell of larger
species at the commencement of egg laying enables egg andfemale accommodation. The shells of female A. nodosus are
considerably more developed than those of smaller species
when spawning commences; five females with ShL ranging
from 54.6 to 62.1 mm (and DML ranging from 31.1 to 38.5 mm)
were found to still be immature (see Finn, 2013, for details).
Immediacy of transformations and reversions. Additional
qualitative support comes from the abrupt nature of shelltransformations and reversions. Shells of female A. nouryi
display obvious precise boundaries between shell formation
types. It is the author’s opinion that the distinct boundaries
between shell formation types indicates that the causal
stimulus acts instantaneously on the female. It is felt thatspawning of eggs would have an immediate effect, requiring
the female to abruptly change the way the shell material is laid

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Figure 35. Preserved female Argonauta nouryi and A. hians with spawned eggs: a, preserved female A. nouryi from the Pacific Ocean (15.2 mm
dorsal mantel length, 18.4 mm shell length, SBMNH 64369) with spawned eggs attached to the axis of the shell; b, preserved female A. hians
from the North West Shelf, Western Australia (28.7 mm dorsal mantel length, 38.9 mm shell length, QM Mo77789) with yellow eggs visible indorsal component of shell. Scale bar = 1 cm.

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J.K. Finn

Figure 36. Images of live female argonauts, Argonauta hians and A. nodosus, demonstrating the effect of spawned eggs on the position of thefemales relative to their shells: a, live female A. hians from Andaman Sea, Thailand, photographed in an aquarium (photo: J. Nabhitabhata, after
Sukhsangchan and Nabhitabhata 2007); b–c, A. nodosus Phillip Bay, Victoria, Australia (photos: R. Kuiter); b, live female argonaut photographedin the wild; c, eggs of same specimen, shown with argonaut removed from shell.

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Argonaut shell variability
down to accommodate the increased volume. If a change in
the growth rate was responsible for the transformation between
shell formation types, it is believed that the transition would be
more gradual and the boundaries in the shell less pronounced.
Argonaut nomenclature and the fossil record.
Misinterpretation of intra-specific shell variation has hindered
the resolution of extant argonaut systematics. Historic
generation of species names based on individual malformed
shells, and shells of different formation types, has createdconfusion and complication. Fortunately, this practice haslargely ceased. The last major erection of new species names
occurred in 1914 when Monterosato proposed three new
species names and one variety based on four shells of A. argo(Monterosato 1914). Interpretation of the fossil record, however,appears to be mirroring the historic approach applied to extant
argonauts. Variation in shell characters is continuing to be used
to designate new fossil argonaut species (Stadum and Saul,
2000), and many have been erected based on single fossilised
shells (e.g. Martill and Barker, 2006). Saul and Stadum (2005)
reviewed the current situation stating: “ten fossil argonautspecies have been placed into four genera based on the absenceor presence of keels and the degree of sculpture” (p. 520). If thesituation is at all similar to that of extant argonauts, great
caution should be undertaken when erecting fossil argonautspecies based solely on shell characters.
Acknowledgements
This study would not have been possible without the generous
financial support of Australian Biological Resources Study,
the Hermon Slade Foundation, the Malacological Society of
London, the Linnean Society of New South Wales, La Trobe
University and Museums Victoria. This study relied heavily
on previously collected argonaut material housed in museum
collections throughout the world. Special thanks go to themuseum staff that facilitated access to this material including(but not limited to) Paul Callomon (ANSP), Bob HamiltonBruce (SAMA), Eric Hochberg (SBMNH), Thierry Laperousaz(SAMA), Ian Loch (AM), Melanie Mackenzie (NMV), Alison
Miller (AM), Darryl Potter (QM), Martina Roeleveld (SAM),
Clyde Roper (USNM), Chris Rowley (NMV), Shirley SlackSmith (WAM), David Staples (NMV), Joanne Taylor (NMV),
Liz Turner (TMAG), Corey Whisson (WAM), Genefor
Walker-Smith (TMAG/NMV), Michael Vecchione (USNM),
Kathie Way (BMNH) and Richard Willan (NTM). Special
thanks go to individuals who provided feedback on various
versions of this manuscript, including: Prema Finn, Eric
Hochberg, Chung-Cheng Lu, Bruce Marshall, Mark Norman,
Richard Young and an anonymous reviewer. Rhyll Plant
deserves special thanks for producing all new illustrations, as
do Rudie Kuiter (Aquatic Photographics) and Charuay
Sukhsangchan (Kasetsart University, Thailand) for providing
additional live photographs, Daniel Geiger (SBMNH) for
assistance with scanning electron microscopy, Richard
Marchant (NMV) for assistance with statistics and Steve
Reynolds (Marine Life Society of South Australia) for
providing essential references.

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